7
Eclipse TFB, V2, Design Guide 310, 11/14/2014
The maximum heat transfer rate of a tube depends on the
temperature of the chamber and how the tube is mounted
inside the furnace or not enclosed.
An enclosed tube has a lower maximum heat transfer rate
than a tube which is tree to radiate in all directions.
Figure 3.2.
Figure 3.3.
Calculate the Maximum Heat Release
Multiply the previously calculated tube surface area by the
maximum heat transfer rate:
Maximum heat release = tube surface area x
maximum heat transfer rate
Compare the heat releases
Compare the required heat release with the maximum
heat release.
If the required heat release is greater than the maximum
heat release, then the number or the size of the radiant
tubes must be increased.
■ Exceeding the maximum heat release will
significantly shorten the tube life.
Determine Efficiency
Decide whether or not you want to use a recuperator. A
recuperator is a heat exchanger which uses heat from the
exhaust to pre-heat the combustion air. The effect of a
recuperator on the efficiency of the system can be
significant, as shown in the table below.
Calculate the Gross Burner Input
Calculate the gross burner input (BTU/hr) with this
formula:
Gross Burner Input = Required Heat Release / Efficiency
Compare the Gross Burner Input
Compare the gross burner input with the maximum tube
input. If the gross burner input is greater than the
maximum tube input from the table, below, then the size
of the radiant tube must be increased.
Exceeding these inputs may result in burner pulsation or
other operational problems.
Sizing Example
Application parameter
• 4 U-tube - 4.5" OD x 75" effective length
• 500,000 BTU/hr total required heat release -
recuperated
• 1650°F chamber temperature
• open radiate angles (not enclosed tubes)
Enclosed tube
Not enclosed tube
1500
1600
1900
3
0
40
50
60
70
Recommended Flux
Maximum Heat Transfer
Rate, Btu/hr per sq.in. of
External Tube Surface
Tube free to radiate
on 3 sides
Tube enclosed
on 3 sides
Furnace Temperature, ºF
1700
1800
CAUTION
Table 3.1 Estimated Gross Efficiency*
Furnace Chamber
Temperature
Without
Recuperator
(Ambient Air)
With
Recuperator
(Preheated Air)
1000°F (538°C) 57% 71%
1300°F (704°C) 51% 68%
1550°F (843°C) 47% 65%
1650°F (899°C) 44% 64%
1750°F (954°C) 41% 63%
1850°F (1010°C) 39% 62%
*Actual efficiency will vary depending on gas type,
recuperator, excess air, piping losses, etc.
Table 3.2 Maximum Tube Input
Tube ID (inches) Maximum Input (1000 BTU/hr)
4 300
5 600
6 900
8 1500
10 2500
12 3500
To Next Page
Loading...
Need help?
General
